The experiments proposed here investigate the brain mechanisms of selective perception in man as revealed in the properties of the sensory-evoked and event-related brain potentials (ERP's) recorded from the scalf. the ultimate goal is to arrive at unified psycho-physiological concepts of selective information processing by considering electrophysiological and behavioral measures of selective performance together as "converging operations" Our studies to dat indicate that separate ERP components behave as reliable sign of distinct modes of selective attention in a wide range of tasks: the N1 wave of the auditory evoked potential (latency 100 msec) indexes an "early" selection process based on the channel characterisitcs of a sound source (e.g., its pitch and spatial position), while the late-positive P3 wave (latency 250-450 msec) indexes a subsequent selection based on the higher-order patterned characteristics of stimuli within the attended channel(s). Further studies are aimed at validating the dichotomous behavior of the N1 and P3 waves for attentive selections of varying complexity in auditory and visual modalities. Quantitative relations between ERPs and signal detection theory measures (d' and Beta) will be obtained throughout. These attention studies will be extended to more complex as meaningful stimuli such as speech passages and natural environmental sounds for which we hope to identify lateralized ERPs corresponding to the hemispheric specialization for language and non-language perceptions. Finally, the psychological specificity and brain origins of the P3 wave will be investigated in patients who have undergone cerebral commissurotomy (split-brain") operations.